This invention relates to an optical recording medium capable of recording information by laser irradiation and to a cyanine dye compound having a specific structure which is useful to make the optical recording medium.
Optical recording discs recordable once with laser light are well known as CD-Rs. Compared with traditional CDs which have pits formed on the substrate by injection molding to record information, CD-Rs are advantageous in that CDs can be produced on a small scale at a reasonable cost and rapidly. The demand for CD-Rs has been increasing with the recent spread of personal computers.
CD-R type recording media typically comprise a transparent disc substrate, a recording layer made of an organic dye (dye recording layer), a reflective layer made of metal (e.g., gold or silver), and a resin protective layer in this order. Writing to an optical disc is carried out by irradiating the optical disc with a laser beam of the near infrared region (usually around 780 nm). The irradiated part of the dye recording layer absorbs the light to generate heat and is thermally deformed (e.g., to form pits) to record the information. The recorded information is reproduced generally by irradiating the optical disc with a laser beam having the same wavelength as the writing beam to detect reflectance difference between the recorded area (the area where the dye recording layer has been thermally deformed) and the non-recorded area (the area where the dye recording layer has not been deformed).
In recent years optical discs called recordable digital versatile discs (DVD-Rs) which are capable of higher density recording than CD-Rs have been proposed. It is known the theory that converging a laser beam diameter is effective to increase recording density and that laser light of shorter wavelength is advantageous for achieving high-density recording because of capability of being converged to a smaller spot size. Based on this principle, DVD-Rs have achieved higher density recording by using visible laser beams (having wavelengths usually ranging 630 to 680 nm), which are shorter than those used for CD-Rs (about 780 nm).
The key to realization of high-density recording to an optical recording medium having a dye recording layer is development of a dye compound possessing excellent recording and reading sensitivity with respect to shorter wavelength light. The present applicant has proposed a DVD-R type recording medium having a cyanine dye-containing recording layer on a substrate in JP-A-10-226170 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). In addition, JP-A-10-181211 discloses trimethinecyanine dyes. Some of this type of cyanine dyes exhibit excellent recording characteristics for laser light of 630 to 680 nm and are promising as a recording dye for high-density recording media.
With the recent broadening of application of CD-Rs and DVD-Rs, the demand for performance stability of optical discs has been getting strict for use under severe environmental conditions, such as considerably varying humidity and temperature conditions. To meet the demand, a dye compound superior to those of JP-A-10-226170 and JP-A-10-181211 in resistance to light and to heat and humidity has been sought for.
In view of the above-described circumstances, an object of the present invention is to provide a cyanine dye compound which has excellent resistance to light and to heat and humidity and is suitable for use as a recording layer dye of an optical recording medium.
Another object of the invention is to provide a recordable optical recording medium capable of recording and reproducing information by laser irradiation, which has provided with a dye recording layer containing the cyanine dye compound so as to be excellent in recording and reading performance and storage properties such as light resistance and durability.
Other objects and effects of the invention will become apparent from the following description.
The above-described objects of the present invention have been achieved by providing:
(1) A cyanine dye compound represented by formula (I): 
wherein A1 and A2 each independently represent a benzene ring or a naphthalene ring; R1, R1xe2x80x2, R2, R2xe2x80x2, R3 and R3xe2x80x2 each independently represent a substituted or unsubstituted alkyl group, provided that at least one of R1 and R1xe2x80x2 represents an alkyl group substituted with a hydroxyl group or an alkoxyl group having 1 to 6 carbon atoms; R4 represents a substituent on the methine chain; q represents an integer of 0 or greater; p represents an integer of 1 to 4; B represents 0, 1 or 2; and Xpxe2x88x92 represents a p-valent anionic tetracyanoquinodimethane compound represented by formula (II): 
wherein R5 represents a substituted or unsubstituted alkylene group; L represents a single bond or a divalent linking group; R6 represents a substituent; p is as defined above; and r is an integer of 0 to 3; and
(2) An optical recording medium comprising a substrate having provided thereon a recording layer capable of recording information by laser irradiation, wherein said recording layer contains a cyanine dye compound represented by formula (I).
The cyanine dye compound of the present invention exhibits excellent resistance to light and to heat and humidity. The optical recording medium of the present invention which contains the cyanine dye compound in the dye recording layer is excellent in recording and reading performance and storage properties such as resistance to light and to heat and humidity.